The present invention pertains to slip-type assemblies for gripping and suspending objects, especially tubular goods such as drill pipe or production tubing. Such an assembly comprises a plurality of circumferentially spaced slip bodies surrounding the locus of the pipe or other object and which in turn are generally surrounded by a generally outer body known as a "bowl." By means well known in the art, the device can be initially emplaced about one section of pipe in a string. The inner sides of the slip bodies carry gripping means, usually in the form of teeth formed on hard metal dies, for biting into and frictionally engaging the pipe. The outer sides of the slip bodies and/or the opposed inner side of the bowl may have surfaces which are inclined radially inwardly and downwardly. Because the slip bodies can move both longitudinally and radially with respect to the bowl, these inclined surfaces serve as camming surfaces. Thus, when the weight of the pipe is let down on the device, so that it tends to move the slip bodies downwardly with respect to the bowl, the camming surfaces urge the slip bodies radially inwardly and into tighter engagement with the pipe, so that the mechanism is self-tightening.
In a typical well drilling operation, two such assemblies are typically employed in, for example, pulling a string of drill pipe or other tubular goods from the well. One such assembly, located at the rotary table, is typically referred to simply as the "slips." The other, which is located above the slips and can travel vertically up and down, is called the "elevator."
If, for example, a string of pipe is being tripped from a well, e.g. to change the drill bit, then with the elevator gripping one of the upper sections of drill pipe, the slips in the rotary table can be released, and the elevator with the pipe carried thereby can be raised farther above the rotary table. The slips in the rotary table can then be engaged with the pipe, so that they support the pipe, and the elevator can be released and moved downwardly to take another bite at a lower point on the drill string. Of course, between the above steps, various sections of drill pipe can be detached from the top of the string as they incrementally clear the elevator.
In general principle, such devices have changed little since they first came into use many years ago. One reason is that, being self-tightening by the very weight of the objects they support, they are considered "fail safe." There are other advantages, including the fact that the devices are relatively simple in construction and operation.
They have not, however, been without problems. Although the dies which actually frictionally engage the pipe or other tubular goods are contoured to generally conform to the curvature of the pipe, pipe damage can still occur. Typically, really objectionable pipe damage occurs when the slip bodies are not properly aligned with the pipe, so that their contours are misfit with those of the pipe. This can occur, for example, if the camming surfaces between the slip bodies and the bowl wear unevenly, if the gripping teeth on the dies wear unevenly and/or become damaged, if the assembly as a whole is not level, and for other reasons. When this occurs, the pipe may not only be deformed, but stress risers can be set up, which may result in premature pipe failure.
This problem is, in a sense, self-exacerbating. Because the slip bodies in prior art devices do not reliably align well enough to provide uniform pressure by all the die teeth, it is common to use sharper teeth so that those which do engage the pipe firmly can support it. However, such teeth are the very ones to cause most damage when the slip bodies are misaligned.
Some prior patents have partially addressed this problem by providing assemblies in which the slip bodies can articulate to adjust for misalignments in longitudinal planes. Examples are U.S. Pat. No. 1,834,316 to McLagan, U.S. Pat. No. 2,061,771 to McLagan, U.S. Pat. No. 2,061,772 to McLagan, U.S. Pat. No. 2,063,361 to Baash. U.S. Pat. No. 2,131,400 to Johnson is similar, but adds a floating hinge to assist in slip body alignment.
The above systems for longitudinal articulation, however, have not been totally satisfactorily. The die surfaces and pipe surfaces may still be misaligned in lateral planes, and the aforementioned damage can still occur. Typically, the only lateral adjustment permitted was by virtue of hinged connections between adjacent slip bodies, which would allow the generally circular locus of the group of slip bodies as a whole to "open up" to disengage the slip bodies from the pipe. It can be appreciated that this type of movement would not allow for proper alignment of the contour of the die with the pipe surface, but on the contrary, could be counterproductive.